The field of the invention relates to an improved apparatus for automatically and precisely aligning two superimposed objects (i.e., aligning a semiconductor wafer or other substrate coated with photoresist material with a mask), and to a method of aligning objects using the apparatus.
In the present day manufacture of integrated circuits, complex circuit patterns are formed on a silicon wafer by photoresist techniques employing a series of contact printings on the wafer. These contacts printings are made from several transparent masks used in succession and in a preselected order. Each successive mask must be accurately aligned with the previous print or prints made on the wafer from the prior masks so that the completed pattern is accurate within a few microns.
The alignment of each mask with the wafer may be accomplished manually by manipulation of the mask over the wafer while the operator observes the mask and wafer through a high power microscope. Alignment may be aided by the use of a pair of spaced-apart detection marks or alignment patterns, for example, crosses, bull""s-eyes, or pinpoint alignment holes, formed on each of the wafer and mask, the pair of alignment patterns on the mask being arranged so that they are superimposed over and aligned with the pair of alignment patterns on the wafer when the wafer and mask are properly aligned.
Apparatus has been proposed for producing alignment of the mask and wafer automatically, thus relieving the operator of this tedious task. One form of such automatic apparatus is described in U.S. Pat. No. 3,497,705 issued Feb. 24, 1970, to A. J. Adler and entitled xe2x80x9cMask Alignment System Using Radial Patterns And Flying Spot Scanningxe2x80x9d. In that system a pair of spaced-apart radial alignment patterns on the wafer is adapted for alignment with a superimposed pair of spaced-apart radial alignment patterns on the transparent mask. The radials of the alignment patterns on the mask are uniformly angularly displaced relative to the radials of the alignment patterns on the wafer when the mask and wafer are properly aligned. A scanning system employing two flying spot scanners scans each of the two pairs of alignment patterns in a circular manner about a center point, measuring the angular distance between the successive radials encountered by the scanning beam. Error signals derived from misalignment of the radials of the alignment patterns on the mask relative to the radials of the alignment patterns on the wafer are utilized to produce relative movement in X, Y, and rotational directions to bring the two pairs of alignment into proper alignment.
Karlson et al., in U.S. Pat. No. 4,052,603, published Oct. 4, 1977, discloses a system for aligning a pattern mask and a photoresist-coated substrate which are separated by an optical element which involves a system for correcting misalignment in the x-or y-directions between the mask and the substrate by using stepper motors to move the substrate in the x-direction or in the y-direction relative to the mask. Rotational misalignment is corrected using a separate mechanism involving gears and levers to rotate the mask relative to the substrate. This apparatus involves use of two separate systems simultaneously to achieve successful alignment, and is rather complicated. It would be desirable to have a system which moves the mask relative to a fixed substrate, rather than moving both the mask and the substrate relative to each other. It would also be desirable to achieve rotational motion and motion in the x-direction or in the y-direction relative to the substrate with a single mechanism, rather than with two separate mechanisms.
The objects of the invention are to provide an improved apparatus for bringing two products into alignment with high precision, and to provide a method for using the apparatus to successfully align two objects.
The invention provides a system for aligning a product, such as a semiconductor wafer or other substrate coated with a photoresist material, and a mask with high precision. The mask has a plurality of detection marks, and the product has a plurality of corresponding detection marks. When the mask and the product are properly aligned, each of the mask detection marks is precisely aligned with the appropriate product detection mark. The system features a product securing surface, where the product securing surface may be made from a transparent having porous sections therethrough adapted to support the product, a means for securing the product to the plate by applying vacuum to the product through the porous sections, and at least two registration pins adapted to fit through registration holes in the product. The invention also features a surface for positioning a mask. Normally, the mask is surrounded by a rectangular mask frame having at least two holes in it. The mask frame is positioned on the mask positioning surface. The mask positioning surface features a holding plate adapted to support the mask frame and at least two alignment pins adapted to fit through alignment holes in the mask frame. In the embodiment described herein, the transparent plate and its porous sections are a recessed portion of the holding plate supporting the mask frame.
After the mask frame is placed on the positioning surface, a number of plungers are activated to resiliently press against two perpendicular edges of the mask frame. These plungers serve to prevent inadvertent or accidental movement of the mask frame and, in the embodiment described herein, are three in number. Computer controlled push plates, also three in number in the present contemplated embodiment, provide the necessary movements to align the corresponding detection marks on the mask frame and the product. In the contemplated embodiment one push plate is set opposite one of the plungers and the remaining pair of push plates are disposed opposite the remaining plungers. Given an arbitrary Cartesian coordinate system imposed on the mask frame and product, sat the single push plate and the corresponding plunger represent movement along the x-axis. Thus, it should be clear that movement along the y-axis is accomplished by synchronous movement of both the other push plates in the desired direction. If rotation is required for alignment, all three push plates may be moved simultaneously using mathematic additions of each individual calculated movement distance.
The system additionally features an imaging means comprising a video camera and an optical microscope adapted to focus an image of the mask and/or the product onto the imaging plane of the video camera. The video camera records an image of each of the detection marks on the product and an image of the detection marks on the mask, and transmits these images to a computer.
The computer then records the position of each of the detection marks on the product; and also records the position of the detection marks on the mask. The computer then compares the positions of the respective detection marks to each other and determines that, to bring the detection marks into alignment with each other the mask needs to be moved in a first direction by a first distance and moved in a second direction by a second distance, and rotated by a defined amount. This can be accomplished be either moving all three simultaneously or by moving them one at a time. It should be noted that the distances involved would be measured in micrometers due to pre-positioning allowed for by the initial positioning pins, as will be discussed further below.